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    <script>
        //1.想要一个变量自己加一
        var num = 1;
        num = num + 1; //++num
        num = num + 1;
        console.log(num); //3
        //2.前置递增运算符 ++ 写在变量的前面
        var age = 10;
        ++age; //类似于age=age+1
        console.log(age);
        //3.先加一后返回值
        var p = 10;
        console.log(++p + 10); //21
    </script>
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